The invention relates to direct current amplifiers in general, and more particularly to a combination of two operational amplifiers mounted as a differential amplifier to respond to a differential signal applied to two input terminals thereof. Such circuitry is particularly applicable to an analog data acquisition system for the detection of low level signals transmitted from remote locations through isolated cables. Such analog data acquistion systems are generally multiplexed with a plurality of measuring points and the acquired analog data are converted into digital data for digital treatment, for instance by a computer.
Because they are often used with narrow band data handling systems with long lines between grounded transducers and amplifiers, operational amplifiers experience alternating current and direct current errors through the amplifying stage, errors which appear as a noise or drift which might severely impair the usually high sensitivity of this type of amplifiers. Signal error due to alternating current mostly is caused by neighboring industrial AC network, at 60 Hertz inducing into the input lines, despite cable isolation and common grounding, an interference signal appearing as an amplified noise with the amplified analog output signal.
It is known to combine two operational amplifiers so that each respond by its non-inverting input to one pole of a single voltage input, the inverting inputs being connected together through a resistor. This is known as a differential amplifier arrangement. If an AC signal is superimposed on the input signal, the high gain characteristic of the arrangement will cause saturation of the amplifying stage by the AC current. Therefore, it is not practical to attempt to eliminate the AC signal by filtering after the amplifying stage. For this reason filtering the AC component has been effected rather at the input side. However, this method of elimination of the AC noise is not satisfying either for several reasons. First, if the filtering action is moderate some AC component remains unfiltered. This fraction, however small at the input, happens to be amplified and while the saturation problem might not occur, substantial noise still will appear at the output. If the filtering action is increased to the maximum, a relatively large resistance has to be inserted in series with the circuitry input in order to generate sufficient AC effect, and this resistance causes a leakage, e.g. DC current to flow, which entails an offset error. Moreover, resistors being by nature exposed to temperature variations, a thermal drift will also occur.
The object of the present invention is to provide a differential operational amplifier which is substantially free from alternating current noise.
Another object of the present invention is to filter out an alternating current component in a direct current amplifier with a small capacitance.
Still another object of the present invention is to substantially attenuate the amplification of a differential amplifier for alternating current noise at a frequency above a predetermined range.
A further object of the present invention is to obtain in a differential amplifier active filtering providing good normal mode rejection of any alternating current component above a predetermined frequency without degeneration when the closed loop direct current gain of the amplifier is being varied.